document how to build on debian-based distros

[enh-google]

this patch names the two packages that aren't likely to be installed already:

diff --git a/README b/README
index f0345ba..63c3a5c 100644
--- a/README
+++ b/README
@@ -34,6 +34,9 @@ make
 make check
 
 The test system requires libmpfr and libmpc.
+To install these on debian:
+
+  sudo apt-get install libmpfr-dev libmpc-dev
 
 For cross build, CROSS_COMPILE should be set in config.mk and EMULATOR
 should be set for cross testing (e.g. using qemu-user or remote access